The present invention relates to a zoom lens particularly to an optical system which is suitable as a photographing lens for a television and can have a focal distance range which can be zoomed easily changed by inserting a lens group for magnification variation into a basic zoom lens.
As a method for shifting a focal distance range of a zoom lens the followings have been known:
That is, (1) a so-called converter system to provide an affocal lens system an angular magnification of which is not 1 in front of a zoom lens, (2) a system to mount an attachment lens between a zoom lens and a camera main body, (3) a system to replace a relay part positioned at an imaging plane side in the rear of a zooming part of a zoom lens with another relay part having different focal distance, (4) a double zoom system containing a zoom system within a relay part, and (5) a system (Japanese Patent Application Sho 48-8118) to insert an attachment lens within a relay lens part consisting of two groups.
Of these systems, in the systems (1), (2) and (3) it is difficult to instantly change a focal distance, and a back focus of the total lens system can not be retained unchanged, further a total lens length will vary. Also the system (4) results in a considerably long total length.
On the other hand in a system of an attachment provided within a zoom lens system, which is the system (5), even if a total length of a basic zoom lens is a bit longer, the change of a focal distance can be done in an instant, and a back focus can be retained unchanged, further a total length of the zoom lens can be retained constant regardless of mounting or dismounting of an attachment lens. Also the number of lenses used will be smaller than that in the system (4), thus cost will be less.
In a conventionally known built-in extender system, two lens group constituting a relay lens are fixed and a lens group to extend a focal distance is mounted between said two groups, thus changing a focal distance while retaining a back focus unchanged. However, when this type of zoom lens is used as a taking lens for a color television camera, a tri-color separation optical system needs to be placed in the rear of an image pick up lens, therefore the back focus distance has to be made sufficiently long. However, since an interval between the front group and the rear group of the imaging lens is made long to have an extender mounted thereat, when an image pick up lens is so designed as to be a bright lens having a small F-number, the lens diameter of the rear group becomes excessively large compared to that of the front group.